Cathode filament



Patented Nov. 2l, 1944 UNITED STATES lParam* oFFlcE CATHODE FILAMENT Carl J. Warnke, Chicago, Application December 16, 1941, Serial No. 423,239

Claims.

' pating area.

An object of the present invention is therefore to provide a filament which will be suitable for cathodes in a variety of purposes and which will be highly dependable in operation and will have extremely long life. In general the object is accomplished by forming the filament, or at least the outer layer thereof, by entwining and preferably braiding suitable wire and coating the wire of the braid with a suitable emissive material. At the present time the wire is preferably tungsten.

Further objects and advantages of the invention will be apparent from the following description and from the drawing, in which: f

Fig. 1 is an elevational view of a length of on form, of filament embodying the present invention, comprising a solid core surrounded by a braid represented diagrammatically.

Fig. 2 is a. cross section of the filament of Fig. l taken approximately on the line 2-2 thereof.

Fig. 3 is a view similar to Fig. 1, representing another form of the invention in which the solid core is omitted.

Fig. 4 is a diagrammatic representation of the cross section of the braid o f Fig; 3, taken approximately on the line 3-.-3 thereof.

Figs. 5 and 6 are views corresponding to Figs. 3 and 4 but showing a third form of the invention in which one braid is superimposed around another.

Fig. 'I is a representation of a helically wound filament which may embody any of the various forms of filament construction of this invention.

Three forms of the invention have been chosen for illustration and description, in compliance with section 4888 of the Revised Statutes, but persons skilled in the art will readily perceive other means for accomplishing the same results. and the claims are therefore to be construed as broadly as possible, consistent with the prior art.

One form of the filament is illustrated in Figs.

l and 2. In this form a core II of solid wire, preferably tungsten, is provided, and a sleeve I2 is carried by the core, preferably in tight contact therewith and welded to the core at its ends as at I3. According to the present invention, the sleeve I2 is a braided wire sleeve, the braid being of the cylindrical or Maypole type which may also be called tubu1ar. Fine tungsten wires are preferably used for braiding the sleeve, the wires being finer than the core I I. As is well known, the cylindrical or Maypole braid is accomplished by winding half of the wires around the core in one direction and half in the other, each Wire crossing alternately inside and outside of those which are wound in the other direction. Crossing over two wires and then under two has been found satisfactory, but this selection is not necessary. The cross section of this braid is seen in Fig. 2. In this figure it may be assumed that the wires I4 all recede in a clockwise direction from the section, while the Wires I5 all recede in a counterclockwise direction from the section. It will be observed that each wire is frequently crossed over by the others. An important result of this is that even if a Wire burns through in the course of use it is held in place and the filament continues in service. Another result is that there is a great expanse of` total area available on which the emissive coating may be effective, although the entire assembly is compact so that the effective radiation is relatively small. In other words, although substantially the entire area of each of the wires is available for emissivity, a relatively small part of this area is fully effective for radiation inasmuch as no surface is fully effective for radiation if part of the radiation therefrom strikes other parts of the filament s o that it is absorbed thereby.

The wires I4 and I5 are preferably braided on the core II so as to have firm contact therewith. The emissive coating may be applied either before braiding or afterwards. The core I I is preiu erably also coated with the emissive material.

Fig. 3 shows another form of the invention in which no core II is provided in the filament, but only a braided tube I8. A cross section of this ,tube is diagrammatically illustrated in Fig. 4.

It will normally be braided' both braids are preferably coated with emissive material. Of course, a solid core could be positioned inside of the inner sleeve if desired, in which case the inner sleeve might be somewhat larger in internal diameter.

Fig, 7 illustrates the preferred shape for the tubular braid for the filament regardless of which of the three forms is used or even if some other form of braided or entwined filament is used. Of course, the shape of the iinished filament will depend entirely on its use and to some extent on the mere preference of the particular manufacturer. The shape shown in Fig. '7 is designed for mercury vapor lamps and the like to conserve heat by positioning the various turns 24 of the helix quite close together so that each absorbs a considerable part of the heat radiated from the others. Turns of 1/4, inch inside diameter have proved satisfactory for ordinary lamps. Other shapes which might be used besides the shape illustrated in Fig. 7 and the straight shape of Fig. 1 are those known as hairpin and conical, but of course the filament may be used in any shape desired.

The sizes of wire used will depend entirely on the uses for which the filament is intended. For a 1D0-ampere airport flood lamp the core Il might be a 1/8 inch rod and the diameter of the wires of the braid might well be as large as .020 to .030 inch or even larger. On the other hand, for a S-ampere lamp the diameter of the wire in the braid would probably be about .002 or .003 inch,'and for some Ipurposes the wire could be even finer. A filament such as the 3-ampere filament described has been tested and indications are that it wouldoperate satisfactorily for about 6000 hours.

The choice between the three forms *cf ills.- ment shown in Figs. 1 to 6 depends somewhat on the use to which they are to be put. In a one or two-ampere lamp a single braided tube would probably be sufncient. With over two amperes two layers would be desirable, either two layers of braid or a layer of braid on a solid core. At still higher values either two layers of braid or two layers oi.' braid and a solid core should be used, or possibly even three layers of braid.

Any desired emissive materials may be used depending partly on the intended use of the cathode. Among those at present preferred for lamps are powdered carbonates and salts of barium or strontium. Compounds of magnesium and cesium might be used, respectively, for rectiiication and photoelectric cells. If applied after braiding, the emissive material should be applied in the form of a very thin solution or fluid mixture, dipping being preferred, using any suitable vehicle or binder and, if desired, an additional solvent for the binder. Solutions' or mixtures of the powdered material in acetone, amyl acetate or alcohol are commonly used and are quite satisfactory. Cellulose acetate can sometimes be used as can vinyl resins, such as dirnethyl-acrylate. A heavier paste can be used if the braiding isv performed in the solution, and in that event the particle size may desirably be fairly large so that the braiding will tend to lock the particles in place.

The coated iilament should be subjected to a conventional activating treatment, sometimes called heat treatment.

Before or after coating, the iilament is` preferably set to the desired shape by applying it to a suitably shaped mandrel and heating it to o-1900 C. in a hydrogen atmosphere. Radio frequency induction may be used for heating it. If it has been coated, this can sometimes serve as the activating treatment, this usually involving the evacuation of the envelope in which it is being heated, the evacuation progressing as the temperature rises.

The braiding of tungsten presents a. problem, and in fact it has been said to be too brittle to braid. I have determined, however, that it will braid satisfactorily if it is not excessively twisted. Of course, it can be braided by hand without twisting. I have also devised a machine for braiding it. Such a machine may be any conventional braiding machine modiiled to avoid twisting the tungsten cumulatively or excessively. For example, the holder assembly of the braiding machine may be so made that the wire carryinl and guiding portions thereof will be rotatable as a unit with respect to their revolving bases. The bases rotate once for each revolution. The holder and wire guide should revolve about the braid without rotating. They could also be so designed that the twisting tension on the wire could be relieved by rotation of the spool, if it is drawn olf from the spool approximately coincident with an axis about which the spool can rotate.

Although the advantages of the invention are best obtained by braiding, they may be obtained also though perhaps to a. lesser degree, by any other method of entwining in which each of a plurality of exposed wires is independent and capable of holding one or more others in place so that the structure will continue to operate satisfactorily even if one wire is burned out. Regardless of how they are entwined, the exposed wires should all be of substantially the same length and all welded to the lead-ln wire, preferably at both ends, so that they will share the load evenly. It is not essential that the inner wires be entwined with one another, since they will be held in Vplace by the outer sleeve.

The core Il may be formed of refractory (sintered) metallic thorium. In that case the lead-ln wires 2B will be separate, and the wires Il and I5 should be welded directly to these lead-in wires so that the current will not have to ilow through the thorium core.

It is also to be understood that thorium wire may be employed, alone or alloyed with other suitable metals, in forming the braided portion of the cathode, or only a portion of the wires forming the braided portion may be o f thorium, as for example the wire I 4. As a result of my invention the thorium or other above described activating compounds or elements are brought into operative combination with the other elements of the cathode as the cathode ages. thereby successively reactivating the cathode.

I claim:

l. A cathode filament including a tubular braid formed of conductive strands tubularly braided, a core of metallic wire within the tubular braid and in contact with the strands thereof, each strand of the braid being wrapped helically around the core, contacting the core at numerous points along its length, intertwined with other strands and welded at its ends to the core, and an emissive material interlocked within the braid and between the braid and core,

2. A cathode filament including a tubular braid formed of conductive strands tubularly braided, each strand being welded at each end to a lead conductor and between the welded ends extending helically in one direction only around a central longitudinally extending zone and being interwoven with strands extending in a different helical directionI and an emissive material within the tubular braid and interlocked within its interstices.

3. A cathode lament including a tubular braid formed of conductive strands tubularly braided, each strand being welded at each end to a lead conductor and between the welded ends extendbeing substantially free from torsional strain.

4. A cathode filament including a coiled tubular braid formed of conductive strands tubularly braided, each strand being welded at each end to a lead conductor and between the welded ends extending helically in one direction only around a central longitudinally extending zone and being interwoven with strands extending in a different helical direction, and an emissive material within the tubular braid and interlocked within its interstices.

5. A cathode lament including a coiled tubular braid formed of conductive strands tubularly braided, a core of metallic wire within the tubular braid and in contact with the strands thereof, each strand of the braid being wrapped helically around the core, contactingA the core-at numerous points along its length, intertwined with other strands and welded at its ends to the core, and an emissive material interlocked within the braid and between the braid and core.

CARL J. WARNKE. 

